Phosphatidylinositol 4,5-bisphosphate clusters act as molecular beacons for vesicle recruitment

Synaptic vesicle exocytosis is mediated by the vesicular Ca(2+)-sensor synaptotagmin-1. Synaptotagmin-1 interacts with the SNARE protein syntaxin-1A and with acidic phospholipids such as phosphatidylinositol 4,5-bisphosphate (PIP2). However, it is unclear how these interactions contribute to triggering membrane fusion. Using both PC12 cells from Rattus norvegicus and artificial supported bilayers we now show that synaptotagmin-1 interacts with the polybasic linker region of syntaxin-1A independent of Ca(2+) via PIP2. This interaction allows both Ca(2+)-binding sites of synaptotagmin-1 to bind to phosphatidylserine (PS) in the vesicle membrane upon Ca(2+)-triggering. We determined the crystal structure of the C2B-domain of synaptotagmin-1 bound to phosphoserine, allowing for developing a high-resolution model of synaptotagmin bridging two different membranes. Our results suggest that PIP2 clusters organized by syntaxin-1 act as molecular beacons for vesicle docking, with the subsequent Ca(2+)-influx bringing the vesicle membrane close enough for membrane fusion.